In 2009, a new railway bridging southern Sweden's Motala Ström River was slated for construction. But then, archaeologists began turning up artifacts there that were thousands of years old. Over the next few years, animal bones, tools made from antlers, wooden stakes, and bits of human skull were found in the bog's lime sediment.

"We had hopes of finding animal bones, but not this rich complex," Hallgren says. "This is quite remarkable."

The findings were published this week in a study in the journal Antiquity, under the witty title "Keep your head high."

Heads Up

At the Kanaljorden excavation site, the 8,000-year-old skulls of nine adults and one infant were found deliberately placed on a densely packed layer of large stones. The skulls did not have jaw bones and two had well-preserved, wooden stakes dislodged inside them. The spikes had been inserted through the large oval openings at the bottom of the skulls, suggesting they had been mounted prior to being deposited in the lake. In one case, a spike was still sticking out of the cranium.

Related: 8,400-Year-Old Skeleton of a Baby Found in GermanyGet a glimpse of the well-preserved, 8,400-year-old skeleton of a baby found in Germany.

Animal bones were also arranged around the skulls, sorted according to the type of creature they belonged to.

"They somehow seem to differentiate between humans and animals but also animals in different categories," Hallgren says.

Two of the human skulls were female, four were male, and two belonged to people between the ages of 20 and 35. The researchers also found a nearly complete infant skeleton, whose tiny bones suggest the individual had been stillborn or died shortly after birth.

The victims' skulls show obvious injuries. There is blunt force trauma near the tops of the heads, and they also appear to have other injuries that show signs of healing. The female skulls have injuries on the back and right sides of the heads, and the male skulls each had one single blow to the top of the head and face.

"These are not people who have been recently smashed in the head and then put on display," Hallgren says. "More than half of them had this healed trauma to the head."

The researchers don't yet know what weapons were used to inflict the damage, and the wounds couldn't be directly tied to the cause of death. DNA analysis is ongoing, but they've found that two of the men are related.

"They are probably not brothers but could be cousins or more distantly related," Hallgren says.

We know that the team uncovered 400 bits of wooden stakes, some of which had been used to mount objects that have long since fallen off. We don't know, however, the reasons for the arrangement.

On Display

The team posits a couple different ideas for why the skulls may have been mounted. It seems like they were deliberately put on display, and it's likely they were buried in another place first. This burial site is small, and since it's the first case of its kind in Mesolithic hunter-gatherer society, it can't be compared to other sites.

"There are not any close parallels," Hallgren says. "We are also working on situating this site in the local and the regional archaeological context."

"We do not have any direct evidence of decapitation," co-author Sara Gummesson of Stockholm University writes in an email to National Geographic. "It is more likely that the crania were separated from the bodies during decomposition."

The blunt force trauma on the skulls could have been inflicted through violence between people, abduction, or some other cause. It's also possible—though unlikely—the injuries could have been inflicted by accident.

Since the male and female skulls show different signs of trauma, the violence could have been deliberately related to gender. It also could have been inflicted in a case of spousal abuse, during raiding and warfare, or due to some sort of cultural practice. Display of the heads could also be a funeral act, meant to honor local community members. Hallgren says the skulls could also be trophies, though he thinks this is unlikely.

More research is being done to solve some of the mysteries around this burial site. The researchers are also excavating nearby bog sites to see if there are any similarities. (Learn about the region's bog mummies.)

"There are many aspects that could be discussed regarding these finds," Gummesson writes, "and I do believe we must keep an open mind about any new results that will come out of the continued work."